5. CHARACTERIZATION OF LOW AND HIGH MOLECULAR-WEIGHT

Paull, C.K., Matsumoto, R., Wallace, P.J., and Dillon, W.P. (Eds.), 2000Proceedings of the Ocean Drilling Program, Scientific Results, Vol. 164

5. CHARACTERIZATION OF LOW AND HIGH MOLECULAR-WEIGHT HYDROCARBONS IN SEDIMENTS FROM THE BLAKE RIDGE, SITES 994, 995, AND 9971

Hermann Wehner,2 Eckhard Faber,2 and Heinz Hufnagel2

ABSTRACT

Sediments from Holes 994C, 995A, 997A, and 997B have been investigated for “combined” gases (adsorbed gas and thatportion of free gas that has not escaped from the pore volume during core recovery and sample collection and storage), solvent-extractable organic compounds, and microscopically identifiable organic matter. The soluble materials mainly consist of polarcompounds. The saturated hydrocarbons are dominated by n-alkanes with a pronounced odd-even predominance pattern that isderived from higher plant remains. Unsaturated triterpenoids and 17ß, 21ß-pentacyclic triterpenoids are characteristic for a lowmaturity stage of the organic matter. The low maturity is confirmed by vitrinite reflectance values of 0.3%. The proportion ofterrestrial remains (vitrinite) increases with sub-bottom depth. Within the liptinite fraction, marine algae plays a major role inthe sections below 180 mbsf, whereas above this depth sporinites and pollen from conifers are dominant. These facies changesare confirmed by the downhole variations of isoprenoid and triterpenoid ratios in the soluble organic matter.

The combined gases contain methane, ethane, and propane, which is a mixture of microbial methane and thermal hydrocar-bon gases. The variations in the gas ratios C1/(C2+C3) reflect the depth range of the hydrate stability zone. The carbon isotopiccontents of ethane and propane indicate an origin from marine organic matter that is in the maturity stage of the oil window.

INTRODUCTION

The sites of Leg 164 were all drilled at the eastern end of the BlakeRidge and southernmost Carolina Continental Rise. Previous investi-gations of Deep Sea Drilling Project (DSDP) Sites 102, 103, 104(Shipboard Scientific Party, 1972), and 533 (Sheridan, Gradstein, etal., 1983) revealed that the Blake Ridge is a major Neogene and Qua-ternary sediment drift that consists of hemipelagic silt- and clay-richcontourite deposits. The objectives of Leg 164 were to increase theunderstanding of all the aspects of generation and occurrence of gashydrates and their impact on the environment in an area where theirexistence had already been suggested by observations on DSDP Leg11 (Claypool et al., 1973) and verified by sampling on DSDP Leg 76(Kvenvolden and Barnard, 1983).

We have characterized the amount and composition of the organicmatter in the drilled sections to explain its possible contribution to theformation of gas hydrates. The investigations were conducted onwhole-round cores from Holes 994C, 995A, 997A, and 997B. One ofthe targets was to look for the occurrence of thermogenic gases. Onlythe deep holes of Leg 164 were sampled to determine the extent of insitu formation of gases during maturation of the sedimentary organicmatter with increasing depth.

Sites 994, 995, and 997 make up a transect of holes that have pen-etrated below the base of gas hydrate stability within the same strati-graphic interval over a short distance of 9.6 km. Even within this dis-tance, small variations in the composition of the organic matter(Rock-Eval data, maceral data) occurred. The ages of the sedimentsdrilled at the three locations are roughly the same and in the range oflate Miocene to Pleistocene

Three lithostratigraphic units were identified in all three holesbased on sediment composition and primary variations in the nanno-fossil and total carbonate contents. The sediments consist mostly ofgreen to gray nannofossil-bearing clays, claystones, and marls of late

1Paull, C.K., Matsumoto, R., Wallace, P.J., and Dillon, W.P. (Eds.), 2000. Proc.ODP, Sci. Results, 164: College Station, TX (Ocean Drilling Program).

2Federal Institute for Geosciences and Natural Resources, Hannover, FederalRepublic of Germany. Correspondence author: [email protected]

Miocene to Holocene age (Paull, Matsumoto, Wallace, et al., 1996).Pyrite is a common accessory mineral and occurs throughout the pro-file, being somewhat less abundant in deeper sections. Microfossils(diatoms, foraminifer, sponge spicules, and chitinous-phosphatic fau-nal relics) constitute the coarsest fraction of the sediments.

TECHNIQUES

After the cores were recovered, they were cut into sections at thecatwalk, and the sediment samples within the plastic liners were deepfrozen in liquid nitrogen immediately. Unfortunately, the coolingchain was interrupted during transport to Germany. For this reasonthe samples reached temperatures above zero for a few days. Theywere kept in a deep freezer (at ~–20°C) again until the laboratory in-vestigation commenced. For the analysis of the nonvolatile organicmatter, the samples were freeze dried and aliquots were impregnatedwith a resin for organic petrographic investigations. After having pol-ished the blocks, maceral composition and vitrinite reflectance weremeasured under the microscope using incident light of 546 nm andfluorescence illumination with blue light irradiation. Due to the scar-city and the small size of the huminite only 2–14 readings per samplewere achieved.

The composition of the structured organic matter is determined bya semiquantitative estimation method that is based on a procedure ap-plied successfully for vegetation surveys (Hufnagel and Porth, 1989).The contents of the main components (liptinite, bituminite, vitrinite,inertinite, and fecal pellets) are given in percent, and the compositionof the liptinite (sporinite, alginite, and faunal liptinite) is similarly ex-pressed. For organic-geochemical analysis, the dried samples werecrushed in a disc mill. Total carbon and sulfur content were analyzedin the untreated sample with a LECO CS-444 analyzer. Prior to totalorganic carbon (TOC) analyses and the determination of 13C/12C ra-tios of kerogens, carbonate carbon was removed with hot 2N HCl(80ºC, 3 hr). Calibration was done with LECO steel rings and ahomemade rock standard. The error associated with the TOC analysisis ±3% relative.

The crushed samples were extracted for 3.5 hr in a Soxtec devicewith acetone/hexane (50:50, v:v) as solvent, along with activated

47

mailto:[email protected]

H. WEHNER, E. FABER, H. HUFNAGEL

copper granules as a sulfur-removing agent. Due to the low amountsof soluble organic matter obtained, no further separation by quantita-tive liquid chromatography was done. After precipitation of the as-phaltenes with n-pentane, the remainder was cleaned up by filteringit over aluminum oxide. Gas chromatographic separation of the non-aromatic fractions was done on a 30 m DB 5 fused silica column in aHewlett Packard (HP) 5890 gas chromatograph connected to a HP5972 mass selective detector (MSD). For the identification of nor-mal- and cyclo-alkanes, mass spectra and retention times were used.

Extraction of hydrocarbon gases from the wet samples was donewith a vacuum-degassing system (similar to that described by Faberand Stahl, 1983). About 150 g of sample material was heated to100ºC in a closed bulb (~1 L), and phosphoric acid was addedthrough a valve. The “combined” gases were extracted from the un-sieved samples, consisting of “free” and “sorbed” hydrocarbon gases(e.g., Faber et al., 1990). Free gases are considered to consist predom-inantly of bacterial methane, thermal gases are mainly found in thesorbed phase, and combined gases are often a mixture of bacterialmethane and thermal hydrocarbon gases (Faber et al., 1997, Whiticarand Suess, 1990).

Gas concentrations are measured using a conventional gas chro-matograph. Data is given in parts per billion by weight (ppbw). Gasratios (e.g., C1/[C2+C3]) are based on volumetric concentrations. Forisotope analyses, methane (C1), ethane (C2), and propane (C3) areseparated on a GC column and oxidized to CO2 and H2O. The CO2is used for stable carbon isotope ratio determination with an isotope-ratio mass spectrometer (Finnigan MAT 251 IR-MS). The isotoperatios are given in the δ-notation and are related to the Peedee belem-nite (PDB) standard. The standard deviations of the isotope valuesfor the sediment gases are in the order of ± 1‰ for δ13C1, δ13C2, andδ13C3 (see Dumke et al., 1989). Minimum sample quantities areabout 1 µL for δ13C1, δ13C2, and δ13C3. Deuterium measurementscould not be performed due to the insufficient gas quantity.

RESULTS

Amount and Composition of the Solvent-Extractable Organic Matter and the Kerogen

The TOC contents of the samples from all three holes increasefrom 0.3% to 0.5% in the uppermost samples to levels around 1.5%at a depth of about 200 mbsf, and remain constant below (Fig. 1; Ta-ble 1). Shipboard and shore-based results correlate despite the factthat quite different methods were used (Shipboard Scientific Party,1996). The total sulfur contents (TS) are around 1% for all samples.No correlation between the TOC content and the TS nor a depth trendin the TS concentrations was observed. Only samples from within thesulfate-reduction zone appear to be depleted in sulfur.

Between 100 and 1000 ppm soluble organic matter (SOM) couldbe extracted from the dried sediments. There is a weak but nonlinearcorrelation to the TOC concentration. Normalized to the TOC con-tent 1%–6% of the organic carbon is soluble with a slight tendencytowards an increasing solubility with increasing TOC contents. How-ever, an increase of the SOM/TOC ratio (ratio is not expressed in Ta-ble 1) with depth that would indicate a maturation effect is absent.

A large part of the extract consists of asphaltenes (up to 22%) andother polar compounds. The nonaromatic hydrocarbon fraction com-prises normal-, iso-, and cyclo-alkanes and alkenes. The n-alkanescontain from 16 to 35 C atoms with a pronounced odd-even-predom-inance in the high boiling range. The alkane n-C29H60 is typically themost prominent compound in the chromatograms, which indicates acontribution of higher land plants to the organic matter (Eglinton andHamilton, 1963), although odd-numbered long-chain n-alkanes havebeen detected also in various species of microalgae (Zegouagh et al.,1998). About 40% of the samples are characterized by n-alkane dis-tribution curves that show a second maximum around n-C17H36,

48

which unequivocally suggests an algal input (Fig. 2A; Blumer et al.,1971). These samples are more concentrated in the Pleistocene–Pliocene part of the profiles. The changes in the relative compositionof the organic matter with increasing age of the sediments is ex-pressed by the change in a variety of hydrocarbon ratios (Fig. 3; Ta-ble 1).

1. The concentration of the isoprenoids pristane and phytane rela-tive to n-C17H36 and n-C18H38 is constant with depth until about400 m. At the final depth, pristane/n–C17H36 ratios as high as 2and phytane/n-C18H38 ratios as high as about 6 were deter-mined, whereas these ratios are below 1 in the near-surfacesamples (Figs. 3A, 3B).

2. The pristane/phytane concentration ratios decrease from levelsaround 1 at the sediment surface to about 0.3 at the final depthin all three boreholes (Fig. 3C).

3. All samples contain thermally unstable hopenes and ß, ß-hopanesin varying amounts (Figs. 2C, 2D; Table 2; Figs. 3D, 3F), whichindicates the immature stage of the sediments. The concentrationof these compounds, relative to that of the stable α,ß-hopanes,increases with depth, which shows that maturation does not playa major role in transforming the organic matter. Otherwise thedepth trend of the ratios plotted in Figures 3D and 3F should bereversed.

4. The odd-even-predominance (calculated for the n-C27H56) in-creases steadily from around 1 to 2 at the seabed to ratios ofaround 3.6 and 3.0 at the final depths of Holes 994C and 995A,whereas it is rather constant at about 2 for the profile of Holes997A and 997B (Fig. 3E). Again, if maturation would havebeen the dominating factor, an opposite depth trend for Holes994C and 995A should occur.

Aside from the features of immaturity present in all samples (odd-even predominance of n-alkanes, unstable hopenes and hopanes),

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Figure 1. Total organic carbon (TOC) vs. depth for Holes 994C, 995A, 997A,and 997B.

LOW AND HIGH MOLECULAR-WEIGHT HYDROCARBONS

Table 1. Organic-geochemical data of samples from Holes 994C, 995A, 997A, and 997B.

Notes: Age = biostratigraphic age (Paull, Matsumoto, Wallace, et al., 1996); C(tot) = total carbon; C(carb) = carbonate carbon [C(tot) – TOC]; TOC = total organic carbon; TS = total sul-fur; Sol = solvent-extractable organic matter; Pri = pristane; C17 = n-C17H36; Phy = phytane; C18 = n-C18H38; OEP 27 = odd-even predominance of n-C27H56 vs. (n-C26H54 + n-

C28H58); hop17 = hop17-(21)-ene; hopane = 17α(H),21β(H)-hopane; β,β-hop = 17β(H),21β(H)-hopane; δ13C kerogen = δ12C/13C-ratio of kerogen (relative to PDB). ND = no data.

Core, section Age

Depth (mbsf)

C (tot) (%)

C (carb) (%)

TOC (%)

TS(%)

Sol(ppm)

Pri Phy

Pri OEP 27

hop17 ß,ß-hop δ13C kerogen C17 C18 hopane hopane

164-994C-1H-2 Pleistocene 2.95 2.41 1.66 0.75 0.46 243 0.48 0.56 0.54 1.18 0.01 0.50 ND4H-4 Pleistocene 29.35 3.33 2.64 0.69 0.89 366 0.59 0.69 1.30 1.60 0.10 1.00 ND7H-5 Pleistocene 59.35 2.05 1.29 0.76 0.91 386 0.56 0.81 0.77 2.42 0.26 1.19 ND10H-5 Pleistocene 79.35 4.30 2.93 1.37 0.74 516 0.45 0.41 1.00 2.75 0.24 2.09 ND12H-5 Pleistocene 98.35 2.42 1.71 0.71 0.85 431 0.45 0.40 0.95 2.54 0.29 1.62 ND15H-5 Pliocene 126.13 3.41 2.64 0.77 1.27 240 0.50 0.55 0.70 2.22 0.34 1.73 ND20X-1 Pliocene 159.85 2.42 1.57 0.85 2.20 325 0.46 0.49 0.51 2.27 0.12 1.59 ND23X-4 Pliocene 189.95 2.53 1.05 1.48 1.37 352 0.62 0.64 0.78 2.46 0.29 1.96 ND26X-1 Pliocene 214.35 2.46 1.33 1.13 1.01 506 0.69 0.69 0.58 2.94 0.37 1.86 ND30X-1 Pliocene 243.15 3.20 1.90 1.30 1.16 318 0.40 0.50 0.54 2.69 0.21 1.72 ND33X-2 Pliocene 273.24 3.92 2.62 1.30 1.04 452 0.40 0.59 0.55 3.21 0.56 2.30 ND38X-1 Pliocene 310.40 3.60 2.11 1.49 1.09 466 0.27 2.47 0.08 1.44 0.68 1.71 ND41X-2 Pliocene 340.40 3.11 1.86 1.25 1.43 365 0.76 0.70 0.29 2.27 0.57 1.03 ND44X-2 Pliocene 369.60 2.81 1.53 1.28 1.08 572 0.60 0.70 0.63 2.68 0.78 1.56 ND49X-5 Pliocene 402.62 3.76 2.23 1.53 0.64 165 0.71 0.55 0.32 2.09 0.92 1.71 ND52X-1 Pliocene 425.95 3.72 2.45 1.27 1.00 470 0.67 1.66 0.28 3.00 1.18 2.18 ND55X-1 Pliocene 444.76 2.92 1.64 1.28 1.18 383 0.75 1.11 0.39 2.97 0.59 0.93 ND58X-1 Pliocene 473.90 2.92 1.57 1.35 1.13 335 0.84 1.12 0.25 2.58 1.93 2.24 ND62X-2 late Miocene 504.30 3.09 1.79 1.30 1.23 816 0.61 1.04 0.50 2.70 1.15 1.72 ND65X-5 late Miocene 529.50 3.13 1.47 1.66 0.96 505 1.25 2.97 0.34 3.10 1.69 2.65 ND69X-3 late Miocene 563.66 3.02 1.46 1.56 0.99 324 2.72 5.24 0.34 3.65 1.48 2.62 ND73X-4 late Miocene 593.07 2.31 1.13 1.18 0.87 415 0.77 1.11 0.32 3.10 1.31 2.22 ND76X-5 late Miocene 624.30 3.37 1.98 1.39 0.18 523 0.65 1.35 0.42 3.03 1.68 2.26 ND79X-4 late Miocene 650.71 3.52 2.02 1.50 0.84 541 1.12 2.91 0.24 3.63 1.72 3.14 ND82X-1 late Miocene 676.05 3.18 1.78 1.40 1.14 360 0.76 1.48 0.36 3.37 1.31 2.11 ND84X-2 late Miocene 696.45 3.28 1.77 1.51 0.99 555 0.70 1.17 0.57 3.31 1.66 2.37 ND

164-995A-2H-1 Pleistocene 3.15 2.35 2.01 0.34 0.12 92 0.46 0.41 0.52 1.95 0.22 1.09 –21.95H-1 Pleistocene 31.65 2.64 2.08 0.56 0.93 198 0.55 0.62 0.60 2.30 0.36 1.58 –20.98H-2 Pleistocene 61.65 1.68 1.15 0.53 1.42 271 0.80 0.85 0.79 1.98 0.23 1.03 –20.812H-3 Pleistocene 91.58 3.93 3.07 0.86 0.91 343 0.60 0.80 0.50 2.54 0.14 1.65 –20.715H-5 Pliocene 124.15 3.99 2.46 1.53 1.63 641 0.61 0.82 0.62 2.67 0.50 2.48 –21.319H-5 Pliocene 152.78 2.92 1.61 1.31 1.03 679 0.55 0.59 1.10 2.53 0.46 2.40 –20.922X-2 Pliocene 177.60 2.25 0.89 1.36 1.31 808 0.51 0.51 1.55 2.28 0.39 1.85 –20.525X-3 Pliocene 208.00 2.57 1.42 1.15 1.00 529 0.58 0.70 0.74 2.16 0.28 1.08 –20.529X-2 Pliocene 235.58 3.30 1.82 1.48 0.94 253 0.55 0.57 0.88 2.27 0.60 1.75 –20.932X-1 Pliocene 263.23 3.44 2.04 1.40 1.04 602 0.52 1.05 0.57 1.69 0.55 2.04 –20.635X-1 Pliocene 291.90 2.65 1.63 1.02 1.13 205 0.52 0.79 0.41 2.76 0.41 1.47 –20.540X-2 Pliocene 331.64 3.31 1.74 1.57 0.90 602 0.41 0.87 0.61 2.40 0.69 1.79 –21.443X-2 Pliocene 361.15 3.05 1.76 1.29 1.19 618 0.57 0.95 0.56 3.31 1.25 2.69 –20.747X-2 Pliocene 389.85 3.71 2.35 1.36 1.07 585 0.50 16.79 0.03 3.13 0.94 2.68 –20.651X-2 Pliocene 418.75 3.88 2.18 1.70 0.97 1052 0.42 0.65 0.94 2.92 1.53 2.11 –20.854X-3 Pliocene 439.60 3.88 2.40 1.48 0.83 810 0.69 1.59 0.54 2.57 1.41 2.71 –20.857X-2 Pliocene 467.10 3.38 1.94 1.44 0.95 650 0.74 2.76 0.24 2.81 1.31 2.46 –20.862X-3 Pliocene 506.56 3.18 1.83 1.35 1.22 796 0.72 0.37 1.99 3.51 1.97 2.45 –21.065X-3 Pliocene 536.00 3.66 2.00 1.66 1.13 898 1.13 3.09 0.38 2.88 2.42 3.34 –20.468X-2 late Miocene 563.30 3.03 1.54 1.49 0.93 750 1.02 2.76 0.50 2.78 1.71 2.78 –21.373X-4 late Miocene 603.99 3.55 1.99 1.56 0.90 853 1.94 5.47 0.38 2.78 1.72 2.59 –21.277X-3 late Miocene 640.27 3.51 2.44 1.07 0.85 237 1.79 4.42 0.53 2.98 1.73 2.38 –20.580X-4 late Miocene 671.42 2.87 1.69 1.18 1.12 413 0.78 1.79 0.25 3.06 1.33 2.35 –21.0

164-997A-3H-2 Pleistocene 15.25 2.50 1.99 0.51 0.67 197 0.49 0.61 0.75 2.59 ND 0.62 ND9H-6 Pliocene 70.35 3.63 3.08 0.55 0.62 241 0.54 0.61 0.85 2.54 ND 1.17 ND12H-2 Pliocene 92.70 3.78 2.38 1.40 1.58 562 0.50 0.61 0.87 2.63 0.55 2.45 ND15H-6 Pliocene 127.35 2.86 1.80 1.06 1.31 405 0.52 0.59 0.86 5.37 0.41 2.05 ND18P-1 Pliocene 146.90 3.24 1.73 1.51 1.42 794 0.53 0.75 0.80 2.14 ND 0.36 ND19H-5 Pliocene 154.63 2.73 1.21 1.52 1.45 614 0.42 0.56 0.79 2.32 0.43 2.08 ND22X-5 Pliocene 179.54 2.42 0.96 1.46 0.98 1048 0.44 0.44 1.11 1.78 0.25 1.21 ND25P-1 Pliocene 202.81 2.14 1.13 1.01 1.18 576 0.67 1.32 1.06 2.15 0.05 0.20 ND26X-3 Pliocene 207.85 2.14 1.33 0.81 0.32 312 1.08 3.86 0.45 2.80 0.70 1.98 ND30X-3 Pliocene 236.24 3.23 1.57 1.66 1.05 1044 0.46 0.56 1.06 7.57 0.57 1.66 ND34X-4 Pliocene 266.78 2.34 1.39 0.95 1.44 360 0.48 0.54 0.72 2.24 0.58 1.62 ND37X-4 Pliocene 294.85 3.33 2.05 1.28 1.02 551 0.52 0.52 0.65 2.91 0.99 2.48 ND41X-1 Pliocene 320.10 3.51 2.18 1.33 0.92 383 0.47 0.52 0.72 3.60 1.05 2.45 ND44X-4 Pliocene 352.60 2.79 1.66 1.13 1.07 501 0.47 0.67 0.80 2.17 0.67 1.66 ND47X-3 Pliocene 379.67 3.60 2.42 1.18 1.00 323 0.52 0.82 0.25 2.26 0.68 1.67 ND52X-4 Pliocene 411.20 3.28 2.16 1.12 1.17 459 0.59 0.64 0.52 2.70 0.90 2.40 ND

164-997B-8X-4 Pliocene 448.80 3.40 2.00 1.40 1.28 312 0.99 2.95 0.18 2.43 1.16 1.95 ND10P-1 Pliocene 462.20 3.15 1.58 1.57 1.17 1017 1.39 3.25 0.49 1.91 0.08 0.18 ND12X-3 late Miocene 475.56 3.64 2.12 1.52 1.04 689 0.88 2.81 0.49 1.06 0.81 1.48 ND16X-3 late Miocene 506.25 3.33 1.77 1.56 1.05 739 1.25 4.13 0.24 2.99 1.41 2.61 ND19X-1 late Miocene 531.00 3.19 1.62 1.57 0.93 269 1.16 2.89 0.31 2.36 1.01 2.13 ND23X-6 late Miocene 567.35 3.26 1.63 1.63 0.95 765 1.94 5.76 0.39 2.69 1.43 2.66 ND27X-5 late Miocene 594.70 2.91 1.50 1.41 0.10 589 0.97 2.33 0.35 1.74 1.38 2.32 ND30X-1 late Miocene 617.50 3.25 1.97 1.28 1.02 648 0.98 3.19 0.45 1.59 1.13 1.53 ND34X-1 late Miocene 646.40 2.25 1.04 1.21 0.89 451 0.96 6.93 0.26 2.71 1.75 2.06 ND37X-1 late Miocene 666.55 4.04 2.76 1.28 0.92 262 0.73 4.42 0.21 2.78 1.46 2.55 ND42X-3 late Miocene 706.07 2.94 1.66 1.28 1.14 732 1.45 4.69 0.44 1.60 1.54 2.24 ND46X-6 late Miocene 739.14 3.02 1.77 1.25 1.00 372 1.30 5.18 0.25 2.41 1.86 2.44 ND

49


several sediment samples from all three holes show an isomerizationpattern of the extended hopanes (22S-compound >22R-compound)that is characteristic of mature organic matter (Fig. 2D; Table 2; com-pounds 11 and 12, 14 and 15, and 16 and 17). The presence of ther-mally stable and unstable compounds could be due to a mixture ofrecycled mature and autochthonous immature organic matter (seechapter discussion). However, because all sediments sampled withthe grease-containing core barrel have the most pronounced maturepattern, an artificial contamination by grease cannot be excluded. Ananalysis of the grease in the same way as the samples has yet to bedone.

Organic petrographic investigations provide further insight intothe composition of the organic matter. In general, the structured or-ganic matter is evenly distributed and not enriched in distinct layers.The particles are around 10 µm in size, and only pollen may exceed100 µm. Three groups of particles—inertinite, vitrinite (synonymous

20 25 30 35 40 45 50 55 60 65

Ion 71.00 (70.70 to 71.70):CO36574.D

[min]

20 25 30 35 40 45 50 55 60 65

Ion 71.00 (70.70 to 71.70):CO36578.D

[min]

m/z 71

m/z 71

994C1H-2

994C23X-4

Pr

20

Ph

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Ion 191.00 (190.70 to 191.70):CO36612.D

[min]

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Ion 191.00 (190.70 to 191.70):CO36573S.D

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995A54X-3

994C7H-5

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16 17151412

11 4

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nsity

Inte

nsity

Inte

nsity

Inte

nsity

A

B

C

D

Figure 2. Mass fragmentograms m/z 71 (A, B) and m/z 191 (C, D) of sam-ples from Holes 994C and 995A. (A) shows algal and higher plant wax con-tribution, whereas (B) is typical for pure plant wax input. Carbon numbers ofthe n-alkanes and the position of pristane (Pr) and phytane (Ph) are given. (C)shows triterpenoid distributions that are typical for immature organic matter,and the triterpenoid pattern of (D) additionally contains mature compounds(compounds 11–17, except 13). Min = minutes, gas chromatography reten-tion time. For peak identification, see Table 2.

50

with huminite), and liptinite—make up the greater part of the visibleorganic matter. Bituminite and faunal remains are of minor impor-tance (Table 3). The description of the organic particles visible underthe microscope follows the nomenclature of Stach et al., (1982).Submicroscopic organic matter (<1 µm) associated with the inorgan-ic matrix may occur, but it cannot be identified.

1. Inertinite: Highly reflecting inertinite is derived mainly fromcell walls of higher plants (mostly trees) and is producedthrough wildfires. The less-reflecting semi-inertinite is de-rived from organic matter that has been oxidized in the watercolumn or in the course of early diagenesis. Both constituentsare evenly distributed throughout the profiles. Additionally,highly reflective graphite bars, probably derived from erodedcrystalline rocks and thin-walled agglomerates, transported as

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OEP 27

994C

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B

1817

Figure 3. A. Pristane/n-C17H36. B. Phytane/n-C18H38. C. Pristane/phytane. D.Hop 17-(21)-ene/hopane. E. Odd-even predominance (OEP 27, 2* n-C27H56/n-C26H54 + n-C28H58). F. 17ß, 21ß hopane/17α, 21ß hopane vs. depth (mbsf).


soot, were observed. Altogether they make up about 50% ofthe particulate organic matter.

2. Vitrinite: Humic material is also derived from terrestrial vege-tation that was degraded during transportation, sedimentation,microbial activity, and inorganic oxidation processes. The par-ticles are fine-grained, elongated, thin-walled, and mostlybrown in color. Their concentration is below 5% of total parti-cles in the uppermost samples, whereas it is up to about 50%in the deeper sections.

3. Liptinite: Two major subgroups were differentiated: (1)sporinite, predominantly consisting of saccate pollen from co-nifers, and (2) alginite, chiefly made up of dinoflagellate cystsand solitary, spiny algae of planktonic origin. Because colo-nial algae of the Botryococcus type (up to 100 µm in size) arerare, fine-grained liptodetrinite, considered to be algal detritus,is dominating. These liptinites (pollen and algae) were less degraded than theinertinites and vitrinites.The composition of the liptinites as a multicomponent groupchanges downhole. The sporinite dominates, or is in equalconcentration to the alginite, in samples down to about 160mbsf, whereas further down alginite is generally the prevailingconstituent of the liptinite.

4. Bituminite: Fluorescing lenses, thin streaks, and agglomeratesof bituminite are present in all samples, but only in very smallamounts. Some samples contain globular or elliptic brown- toyellow-stained bituminite-like particles, about 10 µm in size,which are interpreted as fecal pellets from copepods and sim-ilar crustaceans.

In general, the content of vitrinite increases with depth (Fig. 4),whereas the liptinite content decreases (Table 3). Within the liptinitegroup, marine algae plays a major role below 180 mbsf, whereasabove terrigenous pollen dominate. Taking into account these chang-es in the composition of the particulate organic matter, with broadscattering the content of marine organic matter (algal-derived)amounts to about 30%, whereas that of terrestrial origin is about 70%in all samples (Table 3).

The maturity of the organic matter, expressed as vitrinite reflec-tance and measured on only 2–14 particles from each of the six se-lected samples, is around 0.3%.

Table 2. Compounds identified by gas chromatography/mass spectrome-try in the saturated hydrocarbon fraction of samples from Holes 994Cand 995A.

Note: See Figure 2.

Number in Figure 2 Compound

1 18α(H)-22,29,30-trisnorneohopane (Ts)2 17α(H)-22, 29, 30-trisnorhopane (Tm)3 17α(H),21ß(H)-30-norhopane4 hop17-(21)-ene5 17ß(H),21α(H)-30-normoretane6 17α(H),21ß(H)-hopane7 17ß(H),21α(H)-moretane8 17α(H),21ß(H)-homohopane(22S)9 17α(H),21ß(H)-homohopane(22R)10 17ß(H),21ß(H)-hopane11 17α(H),21ß(H)-dihomohopane(22S)12 17α(H),21ß(H)-dihomohopane(22R)13 17ß(H),21ß(H)-homohopane14 17α(H),21ß(H)-trishomohopane(22S)15 17α(H),21ß(H)-trishomohopane(22R)16 17α(H),21ß(H)-tetrakishomohopane(22S)17 17α(H),21ß(H)-tetrakishomohopane(22R)20 n-C20H4225 n-C25H5230 n-C30H62Pr PristanePhy Phytane

The varying types of organic matter deduced from chemical andoptical investigations cannot be verified by the isotope ratios of thekerogens from the Hole 995A (Table 1). δ13C values range between–20.4‰ and –21.9‰, which is generally believed to indicate marineorganic matter (Degens, 1969; Kaplan, 1975). On the other hand, amixture of type II to type III kerogen (Fig. 5) is indicated by the lowhydrogen indices (HI; mean HI around 200mg HC/gC) from the ship-board Rock-Eval data (Shipboard Scientific Party, 1996). However,mixed type II/type III organic matter in marine sediments often arisesfrom partial oxidation of type II algal material. Therefore, bulk anal-ysis have to be interpreted with caution.

Amount and Composition of the Combined Gases

The concentrations of the combined gases from Holes 994C,995A, 997A, and 997B are plotted vs. depth (mbsf) in Figure 6. Thedata ranges between 73 and 1554 ppbw, 15 and 107 ppbw, and 6 and61 ppbw for methane, ethane, and propane, respectively. Ethane andpropane are constant with depth with the exception of the uppermostsamples where the concentrations are slightly increased. Methane isbelow ≈400 ppbw in the depth range of 200–400 mbsf and below 600mbsf. Slightly higher methane concentrations are found just above200 mbsf and between ~450 and 550 mbsf. In the uppermost samplesmethane values are also slightly higher than below.

The carbon isotope ratios of the combined gases from Holes994C, 995A, 997A, and 997B are plotted vs. depth in Figure 7. Thedata ranges between –67‰ and –38‰, –36‰ and –23‰, and –34‰and –20 ‰ for methane, ethane, and propane, respectively. The iso-tope data of methane indicates a microbial origin of samples withδ13C1 <≈–55‰ and a thermal signature for those samples with δ13C1>≈45‰. Mixing of methane from different sources or microbialmethane oxidation cannot be excluded. Both processes shift the δ13C1pool towards more positive values. No significant trends in the iso-

0

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600

700

800

0 20 40 60

vitrinite (%)de

pth

(mbs

f)

994C

995A

997A+B

Figure 4. Vitrinite content vs. depth (mbsf) for Holes 994C, 995A, 997A, and997B.

51


Table 3. Organic-petrographical data of samples from Holes 994C, 995A, 997A, and 997B.

Note: Columns 1 + 2 + 3 + 4 + 7 = 100% structured organic matter; columns 5 + 6 = 100% liptinite; mOM = marine organic matter; tOM = terrestrial organic matter = % vitrinite + %inertinite + (% liptinite ×=% sporinite/100); FP = fecal pellets; (mOM + tOM) = 100% structured organic matter; R = vitrinite reflectance; P = present.

1 2 3 4 5 6 7 8 9 10

Core, section

Depth (mbsf)

Liptinite (%)

Bituminite (%)

Vitrinite (%)

Inertinite (%)

Spores (%)

Algae(%) FP

mOM(%)

tOM(%)

R(%)

164-994C-1H-2 2.95 85 15 80 20 17 834H-4 29.35 80 2 18 80 20 16 847H-5 59.35 62 3 35 40 60 37 6310H-5 79.35 55 5 40 60 40 22 7812H-5 98.35 30 5 65 40 60 18 8215H-5 126.13 20 80 40 60 12 8820X-1 159.85 15 85 80 20 3 9723X-4 189.95 45 5 50 40 60 27 7326X-1 214.35 30 3 67 25 75 23 7730X-1 243.15 38 P 3 59 20 80 30 7033X-2 273.24 10 P 5 85 10 90 P 9 9138X-1 310.40 25 P 5 70 10 90 23 7741X-2 340.40 30 3 67 10 90 27 7344X-2 369.60 40 P 5 55 20 80 32 6849X-5 402.62 40 P 10 50 10 90 36 6452X-1 425.95 44 8 48 5 95 42 5855X-1 444.70 45 20 35 100 45 5558X-1 473.90 30 P 30 40 5 95 29 7162X-2 504.30 25 P 35 40 100 25 7565X-5 529.50 30 1 30 39 5 95 29 7169X-3 563.66 15 1 54 30 5 95 14 8673X-4 593.07 10 20 70 20 80 8 9276X-5 624.30 10 25 65 100 10 9079X-4 650.70 30 P 15 55 5 95 29 7182X-1 676.05 25 P 30 45 10 90 23 7784X-2 696.45 25 20 55 5 95 24 76

164-995A-2H-1 3.15 40 60 35 65 26 745H-1 31.65 30 10 60 45 55 17 838H-2 61.65 15 55 30 25 75 11 8912H-3 91.58 25 P 50 25 50 50 12 88 0.3315H-5 124.15 25 40 35 50 50 12 8819H-5 152.78 20 50 30 60 40 8 9222X-2 177.60 38 2 2 53 60 40 5 22 7825X-3 208.00 25 2 5 68 20 80 P 22 7829X-2 235.58 30 5 65 60 40 P 12 8832X-1 263.23 35 2 5 58 5 95 P 35 6535X-1 291.90 20 P 10 70 40 60 P 12 8840X-2 331.64 30 P 10 59 30 70 1 22 7843X-2 361.15 15 8 77 50 50 P 8 92 0.3247X-2 389.85 15 P 4 81 30 70 P 11 89 0.3051X-2 418.75 15 P 5 80 20 80 12 8854X-3 439.60 20 1 8 70 20 80 1 18 8257X-2 467.10 15 1 5 79 20 80 13 8762X-3 506.56 10 5 10 75 5 95 15 8565X-3 536.00 10 30 60 20 80 8 9268X-2 563.30 20 4 22 54 35 65 17 8373X-4 603.99 15 2 25 58 20 80 14 86 0.2977X-3 640.27 15 5 25 55 5 95 19 8180X-4 671.42 15 5 20 60 5 95 19 81

164-997A-3H-2 15.25 70 2 4 24 30 70 P 51 499H-6 70.35 65 3 32 12 88 57 4312H-2 92.70 55 5 40 70 30 17 8315H-6 127.35 30 5 65 70 30 P 9 9118P-1 146.90 25 3 72 65 35 8 9219H-5 154.63 20 10 70 65 35 7 9322X-5 179.54 30 10 60 55 45 P 14 8625P-1 202.81 20 8 72 40 60 P 12 8826X-3 207.85 15 P 10 75 35 65 10 9030X-3 236.24 15 2 10 73 35 65 12 8834X-4 266.78 15 10 75 20 80 P 12 8837X-4 294.85 20 1 15 64 30 70 15 8541X-1 320.10 25 3 20 52 30 70 P 20 8044X-4 352.60 20 20 60 15 85 17 8347X-3 379.67 10 P 20 70 60 40 4 9652X-4 411.20 15 2 18 65 20 80 14 86

164-997B-8X-4 448.80 16 4 20 60 20 80 17 8310P-1 462.20 12 6 22 60 10 90 17 8312X-3 475.56 20 2 8 70 40 60 14 8616X-3 506.25 5 P 25 70 50 50 3 9719X-1 531.00 10 P 30 60 10 90 9 9123X-6 567.35 8 1 50 41 30 70 P 6 94 0.2827X-5 594.70 12 2 55 31 5 95 13 8730X-1 617.50 9 P 53 38 10 90 8 9234X-1 646.40 8 52 40 15 85 7 93 0.3137X-1 666.55 14 43 43 10 90 13 8742X-3 706.07 10 1 30 59 5 95 10 9046X-6 739.14 10 P 40 50 10 90 P 9 91

52


tope data of methane are observed with depth, which could be relatedto the depth range of the hydrate stability zone. The more positivelevels in the upper samples can be explained by microbial oxidationof the free methane that was found in high concentrations above thehydrate stability zone (Paull, Matsumoto, Wallace, et al., 1996). Thesmall trend of the ethane and propane isotope data towards lightervalues from bottom to top is difficult to explain genetically. How-ever, because the samples contained large amounts of microbialmethane, especially in the upper part of the holes just below the sul-fate-reduction zone (Paull, Matsumoto, Wallace, et al., 1996), smallamounts of microbial ethane and propane (with δ13C1 values morenegative than –50‰) may also have been formed (Paull et al., Chap.7, this volume) during methanogenesis. Small amounts of these gasesmay still be present in the upper samples after ethane- and propane-oxidation and/or after the desorption from the samples ceased. There-fore, the isotope data of ethane and propane may represent a mixtureof thermal and, especially in the upper samples, microbial gases.

DISCUSSION

Amount and Type of the Organic Matter

The Leg 164 sediments have TOC values that are typical of oceanmargins (1%–2%) and are enriched in organic matter, compared todeep-sea sediments. The reasons might be the additional transport oforganic matter from the nearshore environments to the Blake Ridgeand enhanced preservation caused by rapid burial. Sedimentationrates increase from about 70 m/m.y. (106 yr) within the Quaternaryto about 300 m/m.y. within the late Miocene at Site 994 (Paull, Mat-sumoto, Wallace, et al., 1996). The TOC content follows this in-crease, at least in the upper part of the profile, down to about 200mbsf (Fig. 1).

The mass accumulation rate of TOC, which takes into account theporosity and the sedimentation rate of the sediments (van Andel et al.,1975, Stein et al., 1989), increases 25-fold downcore (Fig. 8A) from< 0.02 g cm–2 k.y.–1 (103 yr) in latest Pleistocene to 0.5 g cm–2 k.y.–1

during the Miocene for sediments from Hole 994C (Paull, Matsumo-to, Wallace, et al., 1996). This increase is paralleled by an increase inthe vitrinite concentration (Fig. 8B) and by a slight increase of theodd-even predominance of the n-C27H56 (Fig. 8C). We believe there-fore that the prevalence of the odd-numbered n-alkanes is caused bythe presence of terrestrial organic matter and not by marine micro-algae (Zegouagh et al., 1998). Both parameters suggest that terrestrial

0

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800

0 100 200 300 400 500 600

oxygen index (mg CO / g C)

hydr

ogen

inde

x (m

g H

C /

g C

)994C

995A

997A+B

alginitetype I kerogen

exinitetype IIkerogen

vitrinitetype III kerogen

2

Figure 5. Rock-Eval data (hydrogen index and oxygen index, from Paull,Matsumoto, Wallace, et al., 1996) of samples from Holes 994C, 995A, 997A,and 997B (source of the fields after Simoneit, 1986).

organic matter occurs more in the older sections. The microscopic in-vestigation of the organic matter, however, reveals a practically con-stant composition of the particulate organic matter along the profile.A more detailed inspection of the data (Table 3) shows that the organ-ic particles of the uppermost sample from Hole 994C consists of lip-tinite (85%) and inertinite (15%). The liptinite of this sample is madeup of 80% spores (terrigenous) and 20% algae (marine). Therefore,altogether, 17% of the organic particles is of marine and 83% of ter-restrial origin. A comparable overall composition was found for thestructured organic matter of the lowermost sample from Hole 994C.However, in this case the organic particles consist of only 25% lip-tinite, which is made up of algae to 95%. Therefore, the marine partof the organic particles amounts to 24% and the terrestrial part to76%. Additionally, around 20% of the terrigenous organic matterconsists of vitrinite, which is not present in the uppermost sample.Obviously, the terms “terrestrial” or “terrigenous,” often taken as asynonym for type III kerogen, should be used with caution. It shouldbe kept in mind that sporinite, a liptinite classified as type II kerogen,is of terrestrial origin.

The liptinites (pollen and algae) were less degraded than the iner-tinites and vitrinites, perhaps because of less transport. It is well es-tablished that during transport, deposition, and early diagenesis of or-ganic matter the more stable terrestrially derived compounds will bepreserved preferentially (Cornford, 1979). Therefore, the organicmatter that has survived and can be analyzed today is only a fractionof the original input, as far as amount and composition is concerned.The observed increase of terrestrially derived organic matter withdepth might be due to the preferential removal of more easily degrad-

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800

994C995A997A+B

dept

h (m

bsf)

Yield (ppbw)

Methane

Ethane

Propane

0 1000 50 100 20 40500

Figure 6. Concentrations of methane, ethane, and propane of the combinedgases from Holes 994C, 995A, 997A, and 997B plotted vs. depth (ppbw =parts per billion per weight). Shaded area marks depth range of gas hydrateoccurrences, deduced from pore-water chemistry (Paull, Matsumoto, Wal-lace, et al., 1996).

53


able organic matter of the liptinite group. Therefore, the terrestriallyderived organic matter that prevails nowadays in the late Miocene/early Pliocene sediments might be a concentrated residuum. Similarresults were achieved by Rullkötter et al. (1986) when looking atPliocene samples from Hole 533A at the Blake Outer Ridge.

The extractable organic matter of the sediments still contains unsat-urated, labile compounds (e.g., hopenes). These compounds are most-ly produced through the microbial reworking of the organic matter anddo not represent the organic matter input. The increase of the relativeconcentrations of the microbially produced hopenes and ß,ß-hopanes(Brassell et al., 1981; Ourisson et al., 1984) downhole also may reflectthe longer time span available for microbial activity. This may also betrue for the increase of the relative isoprenoid concentrations, ex-pressed as phytane/n–C18H38 or pristane/phytane ratios.

Hopenes have been found previously in ODP sediments, such asin Pliocene samples off the coast of Chile (Kvenvolden, et al., 1995);however, no clear depth trend was recognized. Kvenvolden et al.(1995) has suggested three possible mechanisms for the observationof C32–C35 hopane epimer ratios typical of mature organic matter:

1. Incorporation as recycled material,2. Migration of hydrocarbons from greater depth, and3. Mixing of immature and mature components.

However, pollution by grease used for drilling equipment cannot ex-cluded. Further investigations by compound-specific isotope analy-ses might help to solve this question.

dept

h (m

bsf)

0

200

400

600

800

994C995A997A+B

δ13C (‰)

Methane

EthanePropane

-70 -60 -50 -30 -20

-30 -20

Figure 7. Carbon isotope ratios of methane, ethane, and propane of the com-bined gases from Holes 994C, 995A, 997A, and 997B plotted vs. depth(mbsf). Shaded area marks depth range of gas hydrate occurrences, deducedfrom pore-water chemistry (Paull, Matsumoto, Wallace, et al., 1996).

54

The hydrogen indices (Espitalié et al., 1985), which range be-tween 100 and 300 mg HC/gC (Paull, Matsumoto, Wallace, et al.,1996), classify the organic matter as a mixture of type II and type III.In detail, samples from Site 997 have somewhat lower HI levels(around 100) than samples from the other two sites, which is reflectedby the higher content of inertinite and vitrinite in the Site 997 samples(78%), compared to 67% in Hole 994C samples (Table 3). Selectionof organic matter during transport by currents of different intensitieson the crest and on the flanks of the ridge might explain this finding.It should be mentioned, however, that only the upper part of thedrilled profile of Site 997 has been sampled for Rock-Eval pyrolysis.

There is a discrepancy between the isotopic signature of the ker-ogen and the facies type of the organic matter that needs further dis-cussion and investigation. The isotope ratios of the kerogens are all

y = 243Ln(x) + 781

0.0 0.2 04. 0.6

De

pth

(m

bsf

) R= 0.95

A

y = 2.72x + 2.05

MAR of TOC

R = 0.67

C

y = 54.82x + 0.79

0.0 0.2 0.4 0.6

Vit

rin

ite

(%

)

R = 0.62

B

OE

P 2

7

0.0 0.2 0.4 0.60.50

1.00

1.50

2.00

2.50

3.00

3.50

4.00

0

10

20

30

40

50

60

0

100

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300

400

500

600

700

Figure 8. (A) Mass accumulation rate (MAR) of TOC vs. depth, (B) vs. vit-rinite concentration, and (C) vs. odd-even predominance (OEP; 2* n-C27H56/n-C26H54v + n-C28H58). Samples are from Hole 994C.


in the range of –21‰ for the Hole 995A samples. This value is oftenused as an indication for the input of marine plankton that in numer-ous cases is in the range of –18‰ to –21‰ in Miocene to Holocenesediments (Arthur et al., 1985). However, recent studies of the vari-ation of the δ13C ratio of modern phytoplankton in different oceanicenvironments suggest a broader distribution of the δ13C levels from–21‰ to –8‰ (Popp et al., 1997).

Organic petrographic investigations show the prevalence of iner-tinite, vitrinite, and sporinite, which are all of terrestrial origin. Thereare no indications for the occurrence of grasses (C4 plants), whichcould be responsible for relatively heavy carbon isotope ratios (Fry etal., 1977). Additionally, the alkane distribution, dominated mostly byn-C29H60 (not by n-C31H64, which is typical for grasses; Cranwell,1973), suggests higher land plant (C3 plants) input, consistent withthe Rock-Eval deduced occurrence of type II/III kerogens. Therefore,the kerogen isotope ratios should be in the range of –24‰ to –29‰,typical for the C3 photosynthetic cycle.

The results of the organic-petrographic investigations and the car-bon isotope analyses, which seem to be contradictory at the firstglance, might be explained by a mixture of isotopically light terres-trial and isotopically heavy marine organic matter.

Transformation of the Organic Matterin the Sulfate-Reduction Zone

In contrast to the findings of different authors (Vetö et al., 1995;Dean and Arthur, 1989), the hydrogen indices do not correlate withthe TOC values. There is also a lack of correlation between TOC andTS content (Berner and Raiswell, 1983). Both observations indicatethe degradation of the organic matter that may be used for the sulfatereduction. Another reducing agent besides the sedimentary organicmatter could be methane, which migrates in varying amounts frombelow the sulfate-reduction zone. From the linear shape of the sulfateconcentration profile, Borowski et al. (1996) postulated that anaero-bic methane oxidation is a major sulfate-consuming process in sedi-ments from the Blake Ridge.

Several authors (Littke et al., 1991; Vetö and Hetényi, 1991; Lal-lier-Vergès et al., 1993) attempted to quantify the loss of TOC due tomicrobial sulfate reduction by using the sulfur content of the sedi-ments. This might be possible in nonbioturbated sediments, becausethe amounts of H2S that escaped seem to be limited. Stoichiometryshows that 1% reduced sulfur corresponds to a 0.75% loss of TOC(Vetö et al., 1995). However, Blake Ridge sediment bioturbation mayresult in a substantial loss of H2S. Therefore, original TOC contentswould be higher than calculated by the method of Vetö et al., (1995).On the other hand, if methane is the major organic substrate for thesulfate reduction, the loss would be overestimated.

As a rough estimate and assuming that the loss of H2S may com-pensate the nonconsumption of organic matter in the course of sulfatereduction, the concentration of total sulfur is equivalent to the TOCcontent that might have been lost. Therefore, the original TOC con-tent of the Blake Ridge sediments might have been around 2.5%, incomparison to the mean content of 1.5% today.

Further loss of TOC occurs below the sulfate-reduction zone be-cause of the methane generation. This loss cannot be quantified, be-cause the amount of gas generated is not known. Headspace gas mea-surements probably give too low of readings because of the loss ofgas during core recovery.

Paull et al. (1994) have calculated that under ideal conditions atransformation of 0.5% TOC would be necessary to fill 6% of thepore space with gas hydrate. The pore space of the Blake Ridge sed-iments from within the gas hydrate–stability zone may be filled up to9% with hydrates, deduced from direct methane concentration mea-surements (Dickens et al., 1996) and vertical seismic profiling (Hol-brook et al., 1996). These figures are in the same order of magnitude.Therefore, from the quantitative aspect of the available consumableorganic matter, the greater part of the methane now occurring as gas

hydrate could be explained as having been generated in situ by mi-crobial activity. The generation might be an ongoing process becauseviable microbes are present all along the profile (K. Goodman, pers.comm., 1997), as well as metabolizable organic matter (lipids, suchas alkanes). However, the carbon isotope ratios of CH4 and CO2 of thefree gases suggest their migration from depth (Paull et al., Chap. 7,this volume).

Composition of the Combined Gases in Relationto the Gas Hydrate Occurring Zone

No indications of the hydrate-stability zone were found in theconcentration data of ethane and propane (Fig. 6) and in the carbonisotope data (Fig. 7). Only minor changes in methane concentrationswere observed close to the stability zone, and they are not consideredto properly mark the exact depth position of the stability zone in theholes. However, the ratios C1/(C2+C3) plotted vs. depth in Figure 9are mostly below 10 to 15, somewhat higher in the uppermost sam-ples and significantly higher just above and below the hydrate-stability zone. The reason for the higher levels close to the stabilityzone is unknown at present, but a possible explanation might be giv-en: gaseous hydrocarbons, of predominantly microbial origin migratefrom depth to the surface. The gas hydrate occurring zone (GHOZ)may act as a cap rock, causing an elevated concentration of methanebelow the seal. Within the GHOZ, part of the methane is denselypacked into the hydrate structure, part is filling the pore space as freegas. Thus, an anomalous concentration of methane, which cannot bemeasured due to the applied sampling technique described in this pa-per, has to be assumed (Note: the free methane collected with thePCS [Dickens, et al., 1996] has not been considered in this paper).The low methane concentration now observed within the GHOZ isthought to be an artifact, caused by melting of the gas hydrates duringcore recovery, which is accompanied by a total destruction of the sed-iment structure (“soupy texture”) and a more efficient loss of meth-ane than from over- and underlying horizons.

Higher methane yields above the GHOZ may be caused by the oc-currence of hydrates in such a low concentration that their meltingdid neither influence the pore-water chemistry, nor destroy the sedi-ment structure, but nevertheless contributed to the content of thecombined gases. The elevated methane concentrations between ~20and ~80 mbsf are caused by the active methane generation below thesulfate-reduction zone, which adds additional methane to the streamof migration from below.

Ethane and propane are concentrated relative to methane, whichis indicative of thermogenic gases (Fig. 9). The thermal origin ofthese gases is also evident from their carbon isotope data (Fig. 7), ex-cept perhaps for the uppermost samples. Previous investigations ofsedimentary gases have shown that data of sorbed gases are hardly in-fluenced by degassing processes (Faber et al., 1990; Whiticar andSuess, 1990; Faber et al., 1997). Therefore, the carbon isotope datafrom ethane and propane give information on the type and maturityof the corresponding source material. In Figure 10 the carbon isotopedata from ethane are crossplotted vs. the propane data. With the ex-ception of the data from the upper parts of the cores, most data pointsplot along the isotope maturity line shown in Figure 10. This line wasconstructed according to the model, published by Berner and Faber(1996), for ethane and propane derived from an algal kerogen. It isdependent on the original carbon isotopic composition and the matu-rity of the kerogen. However, as the sediments from which the gaseshave been extracted are immature, ethane, propane, and the thermalmethane are presumed to have migrated from deeper, more matureparts of the sedimentary column. Assuming the isotopic compositionof the kerogen to be δ13C = –21‰, as was determined for kerogens ofsamples from Hole 995A, then the maturity of the source rock forethane and propane (Fig. 10) would be in the range between 0.6% to1.0% vitrinite reflectance (VR). This range corresponds to the oilwindow maturity stage, the beginning of which was mentioned for

55


sediments located 1600 m sub-bottom depth (Herbin et al., 1983) atthe Blake Bahama Basin close to the survey area.

Mesozoic and Cenozoic formations were drilled during DSDPLegs 44 and 76 within the Blake Bahama Basin. The organic matterfrom these sediments is mainly of terrestrial origin. However, marineintercalations, equivalents to the Barremian-Cenomanian North At-lantic black shales, have been observed down to 1600 mbsf (Sum-merhayes and Masran, 1983). The petroleum potential of these hori-zons, which contain a mixture of detrital and aquatic organic matter,is good to medium, according to Rock-Eval data (Herbin et al.,1983).If similar sediments exist below 1600 mbsf, the higher maturity of theorganic matter would be responsible for thermal hydrocarbon gener-ation. Long distance migration, though speculative, may explain thespread of the ethane and propane isotope data.

As mentioned above, the combined methane is a mixture of ther-mal and microbial contributions and can be characterized accordingto Figure 11. The data is scattered, as expected for a mixture, butaligns along a mixing line between data of the potential microbial andthermal partners. Because the bulk data in Figure 10 is scatteredaround 0.8% VR (model: kerogen δ13C = –21‰), the isotope value ofthe corresponding methane can be calculated to δ13C1 = –36‰ (ac-cording to the model of Berner and Faber, 1996); the correspondingC1/(C2+C3) is 4 (Fig. 11). Assuming δ13C1 = –64‰ of the residual freemethane and C1/(C2+C3) = 200 for the sediment gases, the mixing line(Fig. 11) can be fitted to the data. Most of the methane of the com-bined gases, between 30% and 90%, have a microbial origin. How-ever, the results shown in Figure 11 should not be overinterpreted,

0

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800

0 50 100

994C995A997A+B

dept

h (m

bsf)

C1 / (C2+C3)

Figure 9. Ratios C1/(C2+C3) of the combined gases from Holes 994C, 995A,997A, and 997B plotted vs. depth. Shaded area marks depth range of gashydrate occurrences, deduced from pore-water chemistry (Paull, Matsumoto,Wallace, et al., 1996).

56

because the combined gases comprise only a small fraction of theoriginally present total amount of gases, and the concentration of thethermal gases (Fig. 6) is lower by some orders of magnitude than thatof the free gases (Paull et al., Chap. 7, this volume). Also, the com-bined gases may have suffered from microbial (methane) oxidationand desorption from the sediment grains.

CONCLUSIONS

Sediments of Miocene to Pleistocene age were recovered fromHoles 994C, 995A, 997A, and 997B, and their gaseous, soluble, andinsoluble organic matter contents were investigated. The downholevariations of the amount and composition of the organic matter arevirtually parallel in all three holes. The sediments are somewhat en-riched in TOC in comparison to most deep-sea sediments. The lip-tinites are dominated by conifer pollen in the upper sections and bymarine algae in the lower ones. Overall, however, an immature ter-restrially dominated, mixed type of the particulate organic matterprevails with an increase of the vitrinite with depth. These downholechanges in the organic facies are mirrored by changes in the compo-sition of the soluble organic matter. Its low stage of maturity is con-firmed by the occurrence of unsaturated triterpenoids and 17ß, 21ß-pentacyclic triterpenoids.

The organic matter of the drilled sections has a δ13Corg = –21‰,which should indicate a prevailing marine facies. However, opticaland chemical investigations suggest the prevalence of a mixture ofthe organic matter: the marine one isotopically heavier, the terrestrialone lighter.

The combined hydrocarbon gases are a mixture of predominantlymicrobial methane and thermal hydrocarbon gases. Indications of thelocation of the hydrate stability zone were only found in the data ofthe gas ratios C1/(C2+ C3). Ethane and propane are inferred to have apredominantly thermal origin. On the basis of their carbon isotope ra-tios and assuming a δ13Corg = –21‰, the corresponding source rockshould have a marine facies and a maturity in the oil window. There-fore, the bulk of the ethane and propane have migrated from below.

The amount of organic matter that may have been present in thesediments originally would have been sufficient to produce enoughmethane to fill the pore space to the observed level. An additionalsupply of methane from deeper sections, however, is inferred fromthe isotope data.

ACKNOWLEDGMENTS

We gratefully acknowledge the invaluable help of the shipboardtechnical staff. We thank C. Paull, Ph. Meyers, and two anonymousreviewers for their critical and constructive suggestions for improve-ment of the manuscript.

REFERENCES

Arthur, M. A., Dean, W. E., and Claypool, G. E., 1985. Anomalous 13Cenrichment in modern marine organic carbon. Nature, 315:216–218.

Berner, U., and Faber, E., 1996. Empirical carbon/isotope maturity relation-ships for gases from algal kerogens and terrigenous organic matter, basedon dry, open-system pyrolysis. Org. Geochem., 24:947–955.

Berner, R.A., and Raiswell, R., 1983. Burial of organic carbon and pyrite sul-fur in sediments over Phanerozoic time: a new theory. Geochim. Cosmo-chim. Acta, 47:855–862.

Blumer, M., Guillard, R.R.L., and Chase, T., 1971. Hydrocarbons of marinephytoplankton. Mar. Biol., 8:183–189.

Borowski, W.S., Paull, C.K., and Ussler, W., III, 1996. Marine pore-watersulfate profiles indicate in situ methane flux from underlying gas hydrate.Geology, 24:655–658.


Brassell, S.C., Wardroper, A.M.K., Thomson, I.D., Maxwell, J.R., andEglinton, G., 1981. Specific acyclic isoprenoids as biological markers ofmethanogenic bacteria in marine sediments. Nature, 290:693–696.

Claypool, G.E., Presley, B.J., and Kaplan, I.R., 1973. Gas analyses in sedi-ment samples from Legs 10, 11, 13, 14, 15, 18, and 19. In Creager, J.S.,Scholl, D.W., et al., Init. Repts. DSDP, 19: Washington (U.S. Govt. Print-ing Office), 879–884.

Cornford, C., 1979. Organic deposition at a continental rise: organicgeochemical interpretation and synthesis at DSDP Site 397, EasternNorth Atlantic. In von Rad, U., Ryan, W.B.F., et al., Init.Repts. DSDP, 47(Pt. 1): Washington (U.S. Govt. Printing Office), 503–510.

Cranwell, P. A., 1973. Chain-length distribution of n-alkanes from lake sedi-ments in relation to post-glacial environmental change. Freshwater Biol.,3:259–265.

Dean, W.E., and Arthur, M.A., 1989. Iron-sulfur-carbon relationships inorganic-carbon-rich sequences, I. Cretaceous western interior seaway.Am. J. Sci., 289:708–743.

Degens, E.T., 1969. Biogeochemistry of stable carbon isotopes. In Eglinton,G., and Murphy, M.T.J. (Eds.), Organic Geochemistry: Berlin(Springer-Verlag), 304–329.

Dickens, G.R., Wallace, P., and Paull, C.K., 1996. In-situ methane quantitiesacross a bottom simulating reflector on the Blake Ridge. 1st MASTERWkshp. Gas Hydrates, Gent, Belgium, 14–15. (Abstract)

Dumke, I., Faber, E., and Poggenburg, J., 1989. Determination of stable car-bon and hydrogen isotopes of light hydrocarbons. Anal. Chem., 61:2149–2154.

Eglinton, G., and Hamilton, R.J., 1963. The distribution of alkanes. In Swain,T. (Ed.), Chemical Plant Taxonomy: London (Academic Press), 187–208.

Espitalié, J., Deroo, G., and Marquis, F., 1985. La pyrolyse Rock-Eval et sesapplications, Partie I. Rev. Inst. Fr. Pet., 40/5:563–579.

Faber, E., Berner, U., Hollerbach, A., and Gerling, P., 1997. Isotopegeochemistry in surface exploration for hydrocarbons. Geol. Jahrb.,103:103–127.

Faber, E., and Stahl, W., 1983. Analytical procedure and results of an isotopegeochemical surface survey in an area of the British North Sea. InBrooks, J. (Ed.), Petroleum Geochemistry and Exploration of Europe.Geol. Soc. Spec. Publ. London, 12:51–63.

Faber, E., Stahl, W.J., Whiticar, M.J., Lietz, J., and Brooks, J.M., 1990. Ther-mal hydrocarbons in Gulf Coast sediments. GCSSEPM Found. NinthAnn. Res. Conf. Proc., 297–307.

Fry, B., Scalan, R.S., and Parker, P.L., 1977. Stable carbon isotope evidencefor two sources of organic matter in coastal sediments: seagrasses andplankton. Geochim. Cosmochim Acta, 41:1875–1877.

Herbin, J.P., Deroo, G., and Roucaché, J., 1983. Organic geochemistry in theMesozoic and Cenozoic formations of Site 534, Leg 76, Blake BahamaBasin, and comparison with Site 391, Leg 44. In Sheridan, R.E., andGradstein, F.M., et al., Init. Repts. DSDP, 76: Washington (U.S. Govt.Printing Office), 481–493.

Holbrook, W.S., Hoskins, H., Wood, W.T., Stephen, R.A., Lizzarralde, D.,and the Leg 164 Science Party, 1996. Methane gas-hydrate and free gason the Blake Ridge from vertical seismic profiling. Science, 273:1840–1843.

Hufnagel, H., and Porth, H., 1989. Organopetrographic studies in theVisayan Basin, Philippines. Geol. Jahrb., 70:349-384.

Kaplan, I.R., 1975. Stable isotopes as a guide to biogeochemical processes.Proc. R. Soc. London B, 189:183–211.

Kvenvolden, K.A., and Barnard, L.A., 1983. Gas hydrates of the Blake OuterRidge, Site 533, Deep Sea Drilling Project Leg 76. In Sheridan, R.E.,Gradstein, F.M., et al., Init. Repts. DSDP, 76: Washington (U.S. Govt.Printing Office), 353–365.

Kvenvolden, K.A., Rullkötter, J., Hostettler, F.D., Rapp, J.B., Littke, R.,Disko, U., and Scholz-Böttcher, B., 1995. High-molecular-weight hydro-carbons in a south latitude setting off the coast of Chile, Site 859. InLewis, S.D., Behrmann, J.H., Musgrave, R.J., and Cande, S.C. (Eds.),Proc. ODP, Sci. Results, 141: College Station, TX (Ocean Drilling Pro-gram), 287–297.

Lallier-Vergès, E., Bertrand, P., Huc, Y., Bückel, D., and Tremblay, P., 1993.Control of preservation of organic matter by productivity and sulphatereduction in Kimmeridgian shales from Dorset (UK). Mar. Pet. Geol.10:600–605.

Littke, R., Baker, D.R., Leythaeuser, D., and Rullkötter, J., 1991. Keys to thedepositional history of the Posidonia Shale (Toarcian) in the Hils Syn-cline, northern Germany. In Tyson, R.V., and Pearson, T.H. (Eds.), Mod-ern and Ancient Continental Shelf Anoxia. Geol. Soc. Spec. Publ.London, 58:311–333.

Ourisson, G., Albrecht, P., and Rohmer, M., 1984. The microbial origin offossil fuels. Sci. Am. 251:44–51.

Paull, C.K., Matsumoto, R., Wallace, P.J., et al., 1996. Proc. ODP, Init.Repts., 164: College Station, TX (Ocean Drilling Program).

Paull, C.K., Ussler, W., III, and Borowski, W.A., 1994. Sources of biogenicmethane to form marine gas-hydrates: in situ production or upwardmigration? Ann. N.Y. Acad. Sci., 715:392–409.

Popp, B.N., Parekh, P., Tilbrook, B., Bidigare, R.R. and Laws, E.A., 1997.Organic carbon δ13C variations in sedimentary rocks as chemostrati-graphic and paleoenvironmental tools. Palaeogeogr., Palaeoclimatol.,Palaeoecol., 132:119-132.

Rullkötter, J., Mukhopadhyay, P.K., Disko, U., Schaefer, R.G., and Welte,D.H., 1986. Facies and diagenesis of organic matter in deep sea sedi-ments from the Blake Outer Ridge and the Blake Bahama Basin, WesternNorth Atlantic. Mitt. Geol.-Palaeontol. Inst. Univ. Hamburg, 60:179-203.

Sheridan, R.E., Gradstein, F.M., et al., 1983. Init. Repts. DSDP, 76: Washing-ton (U.S. Govt. Printing Office).

Shipboard Scientific Party, 1972. Sites 102-103-104—Blake-Bahama OuterRidge (northern end). In Hollister, C.D., Ewing, J.I., et al., Init. Repts.DSDP, 11: Washington (U.S. Govt. Printing Office), 135–218.

Shipboard Scientific Party, 1996. Explanatory notes. In Paull, C. K., Matsu-moto, R., Wallace, P. J., et al., Proc. ODP, Init. Repts., 164: College Sta-tion, TX (Ocean Drilling Program), 13-41.

Simoneit, B.R.T., 1986. Organic geochemistry of black shales from the DeepSea Drilling Project, a summary of the occurrences from the Pleistoceneto the Jurassic. Mitt. Geol.-Palaeontol. Inst. Univ. Hamburg, 60:275-309.

Stach, E., Mackowsky, M.-T., Teichmüller, M., Taylor, G.H., Chandra, D.,and Teichmüller, R., 1982. Stach's Textbook of Coal Petrology (3rd ed.):Berlin (Gebrüder Borntraeger).

Stein, R., ten Haven, H.L., Littke, R., Rullkötter, J., and Welte, D.H., 1989.Accumulation of marine and terrigenous organic carbon at upwelling Site658 and nonupwelling Sites 657 and 659: implications for the reconstruc-tion of paleoenvironments in the eastern subtropical Atlantic through lateCenozoic times. In Ruddiman, W., Sarnthein, M., et al., Proc. ODP, Sci.Results, 108: College Station, TX (Ocean Drilling Program), 361–385.

Summerhayes, C.P., and Masran, T.C., 1983. Organic facies of Cretaceousand Jurassic sediments from Deep Sea Drilling Project Site 534 in theBlake Bahama Basin, Western North Atlantic. In Sheridan, R.E., andGradstein, F.M., et al., Init. Repts. DSDP, 76: Washington (U.S. Govt.Printing Office), 469–480.

van Andel, T.H., Heath, G.R., and Moore, T.C., Jr., 1975. Cenozoic historyand paleoceanography of the central equatorial Pacific Ocean: a regionalsynthesis of Deep Sea Drilling Project data. Mem.—Geol. Soc. Am., 143.

Vetö, I., and Hetényi, M., 1991. Fate of organic carbon and reduced sulphurin dysoxic-anoxic Oligocene facies of the Central Paratethys (CarpathianMountains and Hungary). Geol. Soc. Spec. Publ. London, 50:449-460.

Vetö, I., Hetényi, M., Demény, A., and Hertelendi, E., 1995. Hydrogen indexas reflecting intensity of sulphidic diagenesis in non-bioturbated, shalysediments. Org. Geochem., 22:299–310.

Whiticar, M.J., and Suess, E., 1990. Characterization of sorbed volatilehydrocarbons from the Peru margin, Leg 112, Sites 679, 680/681, 682,684, and 686/687. In Suess, E., von Huene, R., et al., Proc. ODP, Sci.Results, 112: College Station, TX (Ocean Drilling Program), 527–538.

Zegouagh, Y., Derenne, S., Largeau, C., Bardoux, G., and Mariott, A., 1998.Organic matter sources and early diagenetic alterations in Arctic surfacesediments (Lena River delta and Laptev sea, Eastern Siberia), II. Molecu-lar and isotopic studies of hydrocarbons. Org. Geochem., 28:571-583.

Date of initial receipt: 15 April 1998Date of acceptance: 10 March 1999Ms 164SR-225

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-35

-30

-25

-20

-15

-36 -31 -26 -21

994C

995A

997A+B

Model: Kerog = -21 ‰

δ13C-C2H6 (‰)

VR (%)

1.2

0.6

1.1

1.0

δ13C

-C3H

8 (‰

)

Figure 10. Carbon isotope data of ethane and propane from the combined gases compared to isotope/maturity relationships for marine kerogens, derived after the model from Berner and Faber (1996).

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Mixing

Bacterial-64‰/200

Thermal-36‰/4

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90%

100%

δ13C-CH4 (‰)

C1

/ C2+

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Figure 11. Carbon isotope data of methane and the C1/(C2+C3) ratio of the combined gases from Holes 994C, 995A, 997A, and 997B. The microbial proportion of the methane is indicated by the numbers (in percent) at the calculated mixing line.

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5. CHARACTERIZATION OF LOW AND HIGH MOLECULAR-WEIGHT